1. Field of the Invention
The invention relates to distance measurement and determination devices, and more particularly to electronically controlled digital display of distances determined with the aid of an optically active scale under the control of keyboard instructions.
2. Description of the Prior Art
As is well known to those versed in the art of performing measurements of lengths and distances ranging from the order of tenths of millimeters to several feet, a great variety of instruments with various degrees of precision is available. In the order of increasing precision, there are the meter stick or foot ruler, the vernier caliper, the micrometer, and acoustical and electromagnetic wave devices using the principle of wave reflection or used as interferometers. For ordinary measurements of lengths to a precision of tenths of centimeters, the meter stick is used. Although this is the classical and time-honored method of measurement, it is not entirely satisfactory. The user must overcome errors associated with parallax and interpolation between scale divisions. Furthermore, the scale markings are fixed and conversion to other systems of units must be done as a separate operation, introducing additional errors. Previous devices competing with the meter stick have used mechanical switching of electrical current in an electrical resistor to perform measurements of length.
Gallacher et al U.S. Pat. No. 3,973,326 is illustrative of such a prior art device, having a cursor on a movable wand and a resistor extending along the length of the wand. An electrical contact on the cursor contacts the resistor, thereby forming a potentiometer with an output which is variable with cursor position. A digital voltmeter provides an indication of the distance measured. Such devices are cumbersome to use and the mechanical switching mechanisms cause reliability problems. Moreover, prior devices for performing ordinary measurements of length have not been directly coupled to electronic calculating devices so as to provide convenience and the greater power of electronic arithmetic.
The use of light sensing elements, particularly fiber optics, to determine distances in conjunction with counters and stored program computers is disclosed in Rempert U.S. Pat. No. 3,598,978. However, relative movement is required between the object and the light sensing element, and the device does not operate as a replacement for a ruler or meter stick having a scale thereon. Scales are utilized in Zipin U.S. Pat. No. 3,748,043, but only for viewing by photosensors to determine a more accurate measurement by interpolation. A display is provided for the determined distance which is, however, not under operator control with respect to either scale factor or other functions.
Other devices are known for distance measurement by optical means as illustrated by Renner et al U.S. Pat. No. 3,965,340 and Kimura U.S. Pat. No. 3,784,833. Such devices require the use of gratings to effect light measurement. Renner et al, for example, detects a change in light transmission due to a relative displacement between a fixed interference grating (on a fixed caliper) and a movable grating (on a movable caliper). The changed light transmission is detected to provide an indication thereof on an associated calculator. Kimura requires a diffraction grating to effect light measurement utilizing LED's and photodetectors by affixing the detectors directly to the rear of an index grating. Such devices rely on complicated optical instrumentation and do not provide for ordinary scale measurements of distances of either commonplace or arbitrary scale factors.
Lewis U.S. Pat. No. 3,515,888 uses a reticle assembly to determine position change with respect to a fixed reference by using optical gratings for chopping a light beam. Niss U.S. Pat. No. 3,765,764 uses light deflecting means for coordinate measurements. A source projects light onto a movable deflecting means, thence to a further measuring point. Grendelmeier U.S. Pat. No. 3,599,004 determines scale placement utilizing equi-spaced photocells. A differential amplifier is used to permit measurement of displacements less than the cell size. Such devices, while utilizing light sensing means do not provide a portable means, under operator control, for obtaining distance measurement comparable to ordinary meter-stick manual measurement, nor do the devices provide for variation of a scale factor under operator control. Moreover, the prior art devices do not contemplate the use of an optically active scale for distance setting and measurement.